1. Field of the Invention
This invention relates to a beam control circuit to be applied to an image pick-up tube, and particularly to a beam current control circuit for a pick-up tube having a diode gun.
2. Description of the Prior Art
It is known that there are two types of image pick-up tubes, in which a triode gun type image pick-up tube operates with lower control electrode potential than cathode electrode potential, while a diode gun type image pick-up tube operates with higher control electrode potential than cathode electrode potential.
Namely, when the image pick-up tube having a triode gun is in operation, a negative potential bias of +30V.about.-100V is supplied to a control electrode relative to a cathode potential bias of about zero volts. On the other hand, when the image pick-up tube having a diode gun is in operation, a positive potential bias of +10V.about.+20V is supplied to a control electrode relative to the cathode potential bias of about zero volts.
Accordingly, in case of the image pick-up tube having the diode gun, there flows a control electrode current under operating condition, while there flows no control electrode current in case of the image pick-up tube having the triode gun.
As shown in U.S. Pat. No. 3,975,657, a beam control circuit applied to the image pick-up tube having the triode electron gun by using an excess beam control technique is known. As described in the above-mentioned U.S. Pat. No. 3,975,657, a beam current of the amount of a pick-up tube has to be dynamically controlled in response to an incident light amount in order to maintain high resolution image.
The idea proposed in the above-mentioned U.S. patent is to keep an excess beam current (which can also be referred to as a return beam current) constant in any event. But this patent only describes such control circuit for the image pick-up tube having a triode gun.